Preparation of aluminum nitride



United States Patent 3,307,908 PREPARATION OF ALUMINUM NITRIDE VictorMandorf, Jr., Olmsted Falls, Ohio, assiguor to Union CarbideCorporation, a corporation of New York No Drawing. Continuation ofapplication Ser. No. 255,650, Feb. 1, 1963. This application Aug. 16,1965, Ser. No. 480,091

9 Claims. (Cl. 23192) This application is a continuation application ofcopending application Serial No. 255,650, now abandoned, entitledPreparation of Aluminum Nitride, filed February 1, 1963, which is acontinuation-in-part application of application Serial No. 822,425, nowabandoned, filed June 24, 1959.

The invention relates to a process for preparing aluminum nitride.

The simplest method for preparing aluminum nitride, at leasttheoretically, is to heat aluminum metal in the presence of nitrogen;however, this method is more diflicult than it first appears. Sincealuminum melts at about 660 C. and the reaction of aluminum and nitrogenbegins in general at about 800 C., the aluminum will melt and coalesceinto a pool before the reaction begins, and this prevents an intimatecontact of the reactants. Thus, very low yields are obtained, and thepurity of the product aluminum nitride is low because of the largequantities of unreacted aluminum present.

Another method known in the art for preparing aluminum nitride entailsarcing aluminum electrodes in a nitrogen atmosphere. This methodproduces a material of high purity, but it has not found wide acceptancebecause of its very low efficiency in terms of production rates.

Other known preparative methods for aluminum nitride are generallyunsatisfactory especially on a commercial scale because they eitheryield a product of low purity or have low efficiencies.

Accordingly, the main object of the invention is to provide a processfor preparing aluminum nitride, which process is capable of a high rateof yield and an extremely pure product.

Briefly, the above object is attained by a process which comprisesmixing finely divided aluminum metal with a carrier material, and thennitriding the aluminum in the mixture at a temperature above about 800C. by providing a nitriding atmosphere free from oxygen around themixture and then heating the mixture. The carrier material preventscoalescence of the aluminum metal and thus allows the nitridingatmosphere to impinge upon a large surface area of aluminum. A rapid andcomplete reaction results.

The nitriding atmosphere should be inert to aluminum nitride and can becomposed of any materials as long as it contains nitrogen available forthe nitriding reaction but does not contain materials, such as oxygen,water, cyanogen, hydrogen sulfide, and carbon dioxide, which interferewith the nitriding reaction or the nitrided product to form undesirableimpurities such as oxides, carbides, and sulfides. Nitrogen, ammonia,and mixtures thereof are preferred; however, relatively small amounts ofhydrogen and/or carbon monoxide may be present in the nitridingatmosphere without undesirable effects. Pure nitrogen is preferred forthe production of an extremely pure product.

The carrier material employed in accordance with the present inventionis selected from the group consisting of aluminum nitride, aluminumfluoride, and mixtures thereof. The carrier material should obviously besubstantially pure to minimize the introduction of unnecessaryimpurities into the reaction mixture, and the amount of the carriermaterial present in the total mixture of it and particulate aluminummust be at least about 30 percent by weight with the remaining portionaluminum, i.e., up to about 70 percent. At higher concentrations ofaluminum, the aluminum tends to coalesce, thereby affecting the reactiondeleteriously. The preferred reaction mixture consists of from about 30to about 60 percent by weight aluminum and from about 70 to about 40percent by weight carrier material. With higher concentrations ofcarrier materials, even up to 99 and higher percent, a product of highpurity can be produced, but the total amount of product prepared perbatch will obviously be lower. If aluminum fluoride is used as, or aspart of, the carrier material, it can be easily removed from the productby heating the mixture of product and carrier material to at least about1300 C., the sublimation temperature of aluminum fluoride. If anyaluminum nitride is present as carrier material, there is of course noneed to separate it from the product aluminum nitride. Thus, a productof high purity can be obtained since the carrier material permits asubstantially complete and rapid reaction and since aluminum fluoridecan be easily removed from the reaction mixture when necessary ordesirable.

The fact that aluminum fluoride sublimes rather than melts is a greatadvantage, and a strong recommendation for its use as a carriermaterial. In addition to this characteristic, aluminum fluoride has afairly high vapor pressure at the reaction temperature of aluminum andnitrogen. Thus, as the exothermic reaction proceeds, the reaction ishelped by the increased porosity and surface area caused by partialvaporization of the aluminum fluoride. Furthermore, the vaporization ofthe aluminum fluoride removes energy from the reaction mixture and thushelps to control the exothermic reaction.

The reaction of aluminum and nitrogen begins at a reactant temperatureof about 800 C. and proceeds as follows:

800 C. 2A1 N: 2A1N l28 KOal The highly exothermic nature of the reactioncan be controlled by regulating the nitrogen flow into the reactionmass, and can be used as a guide to indicate the completion of thereaction. At about 800 C., the reaction will supply enough heat tomaintain the temperature necessary for reaction; therefore, an externalsource of heat is usually unnecessary after the reaction mixture reachesabout 800 C. At the completion of the reaction, the temperature willdrop, thereby indicating the reaction is complete. The reaction ispreferably conducted at temperatures between about 800 C. and about 1300C. to prevent the possibility of an uncontrolled reaction which may leadto excessive sublimation of aluminum nitride, coalescence of aluminum,and a resulting decrease in the reaction efficiency. At highertemperatures, the reaction tends to become uncontrollable, although areaction at these temperatures is feasible. Reaction temperaturesbetween about 800 C. and about 1100 C. have proved to be the mostsatisfactory for obtaining a product of excellent quality.

In the practice of the invention, aluminum powders having averageparticle sizes of 20, 30, and microns have been successfully employed.These particle sizes have no effect on the completion of the reaction,since in no case was there free aluminum in the reaction prodnet, but analuminum powder having an average particle size of 120 microns gave abetter yield and a product of higher purity than an aluminum powder ofabout 20 to 30 microns. This was probably the result of the powder of120 microns having a lower percentage of combined oxygen because of asmaller surface area when compared to the finer aluminum powders. Thus,an aluminum powder of about 120 microns is preferred. This in no waylimits the invention, however, since particles of almost any size willbe operative as long as they can be considered as a powder, although thequality of the product may vary somewhat.

The particle size of the carrier material must be fine enough to preventthe aluminum powder from coalescing nitride and as free as possible ofoxygen and other materials Which generally interfere with nitriding andthe formation of aluminum nitride.

Although the carrier material may consist of aluminum fluoride, aluminumnitride, or mixtures thereof, there are certain advantages gained byusing either one, the other, or mixtures of the two. The presence ofaluminum nitride in the carrier material results in a soft reactionproduct, and this elfect varies with the amount of aluminum nitride inthe carrier material. With aluminum fluoride alone as the carriermaterial, the reaction product is a hard sintered cake. This effect isshown in Table I below:

Table I.Eflect of Carrier Material Composition on Reaction ProductBlends Material Al to mesh), Wt. Percent AlN (micro milled), Wt. PercentAlF (micro milled), Wt. Pcrcent Reaction Temp, C 800%,100 O- AtmosphereNitrogen Nitrogen. Stabilization 'lemp 1,900 1 9 1,900.

Atmosphere Argon Argon Argon.

Remarks Reaction product is a Same as 1, except that the Reactionproduct was Reactlou product was hard, white sintered cake. Beforemilling, the material had to be crushed in a jaw crusher.

reaction product crushed more easlly.

soft enough to be crushed easily with mortar and pestle.

After the reaction is complete, the reaction mixture of aluminum nitrideand carrier material is heated to at least about 1300 C. to sublime thealuminum fluoride if it is present in the carrier material and if itsremoval is desired. Furthermore, the mixture is preferably heated to atleast about 1900 C. and maintained at this temperature for a time up toseveral hours to stabilize the product aluminum nitride if this isdesirable. Aluminum nitride produced at temperatures below about 1400 C.tends to be subject to rapid hydrolysis by moist air. Various degrees ofstability can be effected by varying the time and/ or the temperature ofthis stabilization step. In general, the stabilization temperature ispreferably between 1700 C. and 2100 C., and the time is between 5minutes and 3 hours, depending on the type of product desired. Atemperature of about 1900 C. and a time of one hour has proved to be themost satisfactory for a product of good quality. Higher temperatures andlonger times improve the product stability only slightly, and can be adisadvantage in that excessive crystal growth in the product issometimes promoted.

The heating step for stabilization can be conducted in a nitrogenousatmosphere, but it is preferably conducted at least during the latterpart, e.g., during temperatures above about 1900 C., in an atmosphere ofan inert gas, such as argon, neon, helium, and the like. An inertatmosphere will help to prevent erosion of the furnace walls, and at thesame time may permit the removal of any aluminum oxide (A1 0contamination present in the product aluminum nitride. The eliminationof nitrogen in the atmosphere allows a small amount of aluminum nitrideto decompose to maintain equilibrium, thereby providing free aluminumwhich can react with A1 0 contamination to form a volatile oxide. Thereactions are illustrated below:

1900 C. A1 03 4A1 ZAhO (vapor) In general, a carrier material consistingof about 15 to about 50 percent by weight aluminum fluoride and about 50to about percent aluminum nitride is preferred. Such a compositionprovides the best final product for most purposes. A carrier materialconsisting of by weight about 75 percent aluminum nitride and about 2 5percent aluminum fluoride is preferred for production of a generallyusable product on a commercial scale. A reactant mixture consisting ofby weight about 43 percent aluminum and about 57 percent of thisparticular composition of the carrier material is generally preferredfor commercial production. This reactant mixture consists of about 43percent aluminum, about 43 percent aluminum nitride, and about 14percent aluminum fluoride.

The following example illustrates the invention more specifically:

EXAMPLE I 1800 grams of aluminum powder having an average particle sizeof microns, 1800 grams of aluminum nitride powder of about 40 microns,and 600 grams of aluminum trifluoride powder of about 40 microns wereblended with nails in a tumbling barrel. The blend Was placed in agraphite reaction vessel having a perforated bottom and fitted with aninlet tube to permit the introduction of nitrogen into the vesselthrough the perforated bottom and then through the blend. The reactionvessel was placed in a resistance heated graphite tube furnace having anitrogen atmosphere therein.

Nitrogen was fed into the reaction vessel through the inlet tube at arate of about 10 cubic feet per hour, and the furnace was rushed to atemperature of 825 C. to 850 C. at which the reaction wasself-sustaining. The reaction rate was regulated to maintain atemperature below 1100" C. by adjusting the amount of nitrogenintroduced into the reaction vessel. The reaction temperature droppedbelow 825 C. as the reaction reached completion.

The furnace temperature was then raised slowly to 1900 C. and held therefor one hour to stabilize the aluminum nitride. When the temperaturereached 1500" C. during this heating step, the nitrogen atmosphere wasreplaced with an argon atmosphere by feeding argon at a rate of 1 cubicfoot per hour into the reaction vessel.

The furnace was then allowed to cool under the argon atmosphere, and theproduct was removed.

The product yield was 98.5 percent of the theoretical amount, and thepurity of the material was 97 percent based on the amount of nitrogenpresent. Chemical analysis showed by weight 65.6 percent aluminum, 33.0percent nitrogen, and 0.19 percent carbon. The particle sizedistribution of the product is shown in Table II below:

Table II.-Particle size distribution of aluminum nitride product Percentof product Diameter in by weight having microns: a smaller diameter 84100 36 90 17.5 80 12.0 70 9.7 60 8.4 50 7.2 40 6.1 30 5.0 20 3.8 3.0 51.8 0

It will be appreciated by those in the art that the process of theinvention is a single step process in that the reactant mixture chargedinto the furnace is removed as the desired pure final product.

What is claimed is:

1. A process for preparing aluminum nitride, which process comprisesforming a mixture consisting of finely divided aluminum in an amount offrom about 30 to about 60 percent by weight and from about 70 to about40 percent by weight finely divided carrier material selected from thegroup consisting of aluminum nitride, aluminum fluoride, and mixturesthereof, providing around said mixture a nitriding atmosphere inert toaluminum nitride and free from oxygen and other materials whichinterfere with nitriding, said nitriding atmosphere consisting of atleast one member selected from the group consisting of nitrogen andammonia and heating the mixture to at least about 800 C. while undersaid atmosphere, thereby nitriding said aluminum in said mixture to formaluminum nitride.

2. The process defined in claim 1, wherein said nitriding atmosphere isnitrogen.

3. The process defined in claim 2 wherein said carrier material consistsof from about to about 50 percent by weight aluminum fluoride and fromabout 50 to about 85 percent by weight aluminum nitride.

4. The process defined in claim 1 wherein said mixture consists of byweight about 43 percent aluminum, about 43 percent aluminum nitride, andabout 14 percent aluminum fluoride.

5. A process for preparing aluminum nitride, which process comprisesforming a mixture consisting of finely divided aluminum in an amount offrom about to about 60 percent by weight and from about 70 to aboutpercent by weight finely divided carrier material selected from thegroup consisting of aluminum nitride, aluminum fluoride, and mixturesthereof, providing around said mixture a nitriding atmosphere inert toaluminum nitride and free from oxygen and other materials whichinterfere with nitriding, said nitriding atmosphere consisting of atleast one member selected from the group consisting of nitrogen andammonia heating said mixture to a temperature between about 800 C. andabout 1300 C. while under said atmosphere, thereby nitriding saidaluminum in said mixture to form aluminum nitride, and finally heatingsaid mixture to a temperature of at least about 1700 C. under anatmosphere inert to aluminum nitride and free from oxygen and othermaterials which interfere with nitriding, thereby subliming any aluminumfluoride present and stabilizing the reaction product.

6. The process defined in claim 5 wherein said nitriding atmosphere isnitrogen.

7. The process defined in claim 5 wherein said carrier material consistsof from about 15 to about 50 percent by weight aluminum fluoride andfrom about 50 to about 85 percent by weight aluminum nitride.

8. The process defined in claim 5 wherein said mixture consists of byweight about 43 percent aluminum, about 43 percent aluminum nitride, andabout 14 percent aluminum fluoride.

9. A process for preparing aluminum nitride, which process comprisesforming a mixture consisting of finely divided aluminum in an amount offrom about 30 to about percent by weight and from about to about 40percent by weight finely divided carrier material selected from thegroup consisting of aluminum nitride, aluminum fluoride, and mixturesthereof, providing around said mixture a nitrogen atmosphere, heatingsaid mixture to a temperature between about 800 C. and about 1300 C.while under said atmosphere, thereby nitriding the aluminum in saidmixture to form aluminum nitride, and then heating said mixture to atemperature between about 1900 C. and about 2100 C. under an atmosphereof an inert gas at least during temperatures above 1900 C., therebysubliming any aluminum fluoride present and stabilizing the reactionproduct.

References Cited by the Examiner UNITED STATES PATENTS 741,396 10/1903De Chalmot 23l91 X 1,198,965 9/1916 Serpek 23192 1,233,926 7/1917 Serpek23192 1,803,720 5/1931 Miner 23192 2,835,566 5/1958 Perieres et al 23192X 2,929,126 3/1960 Bollack et al. 23-192 OTHER REFERENCES Long et al.:Aluminum Nitride, A Refractory for Aluminum to 2000 C., J. AmericanCeramic Society, vol. 42, No. 2, pp. 53-59, 1959.

Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry,Long-mans, Green & Co., NY. 1928, vol. VIII, pp. 111-114.

OSCAR R. VERTIZ, Prin-zary Examiner.

J. J. BROWN, Assistant Examiner.

9. A PROCESS FOR PREPARING ALUMINUM NITRIDE, WHICH PROCESS COMPRISES FORMING A MIXTURE CONSISTING OF FINELY DIVIDED ALUMINUM IN AN AMOUNT OF FROM ABOUT 30 TO ABOUT 60 PERCENT BY WEIGHT AND FROM ABOUT 70 TO ABOUT 40 PERCENT BY WEIGHT FINELY DIVIDED CARRIER MATERIAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM NITRIDE, ALUMINUM FLUORIDE, AND MIXTURES THEREOF, PROVIDING AROUND SAID MIXTURE A NITROGEN ATMOSPHERE, HEATING SAID MIXTURE TO A TEMPERATURE BETWEEN ABOUT 800*C. AND ABOUT 1300*C WHILE UNDER SAID MIXTURE TO FORM ALUMINUM NITRIDE, AND THEN HEATIN SAID MIXTURE TO A TEMPERATURE BETWEEN ABOUT 1900*C. AND ABOUT 2100*C. UNDER AN ATMOSPHERE OF AN INERT GAS AT LEAST DURING TEMPERATURES ABOVE 1900* C., THEREBY SUBLIMING ANY ALUMINUM FLUORIDE PRESENT AND STABILIZING THE REACTION PRODUCT. 